High heat radiation composite and a method of fabricating the same

ABSTRACT

The present disclosure provides a high heat radiation composite material including a hybrid filler comprising expanded graphite filled with expandable polymeric beads, and a fabrication method thereof. In the method, a dispersion solution is prepared by dispersing expandable polymeric beads in ethanol. Expanded graphite is immersed in the dispersion solution, and heat-treated to remove ethanol, thereby producing the hybrid filler. The hybrid filler is dispersed into the matrix polymer via an extrusion/injection process, thereby producing the composite material.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Divisional Application of U.S. patent applicationSer. No. 13/540,842, filed Jul. 3, 2012, which claims under 35 U.S.C.§119(a) the benefit of Korean Patent Application No. 10-2012-0024049filed Mar. 8, 2012, the entire contents of which are incorporated hereinby reference.

BACKGROUND

-   -   (a) Technical Field

The present invention relates to a high heat radiation compositeincluding a hybrid filler comprising expanded graphite filled withexpandable polymeric beads, which is dispersed in a matrix polymer, anda fabrication method thereof. More particularly, it relates to a highheat radiation composite including a hybrid filler comprising expandedgraphite filled with expandable polymeric beads that has beenheat-treated and dispersed into a matrix polymer by anextrusion/injection process. The composite has better heat radiationcharacteristics than a typical heat-radiating composite, overcomesthermal anisotropy, and may be utilized as a material for battery casesand housing and plates interposed between pouch cells in a cell moduleof a battery system for an electric vehicle.

-   -   (b) Background Art

Generally, thermal runaway is a phenomenon that hinders the efficiencyand reliability of batteries due to local temperature differences orhigh heat caused by high-speed charging, high power, and repeatedcharging occurs in batteries for electric vehicles. Thermal runaway mayresult from a deficiency or lack of external thermal diffusion capacitycompared to the internal heat generated by batteries.

Generally, materials that are being used for battery cases and housingshave a mineral filler, e.g., an incombustible filler is filled in aplastic base material such as PC+ABS, PA and PP by 20 wt % to 30 wt %.These materials provide beneficial characteristics such as flameresistance, chemical resistance, insulation, and durability. However,these materials do not provide good heat radiation characteristics. Inthe case of heat radiation composites that are being developed, thermalanisotropy generally occurs due to the orientation of a filler in aninjection process; however, there is a significant limitation inachieving high heat conduction in such composites because a heattransfer resistance occurs at the interfaces between components such as,for example, filler and resin.

Generally, high heat radiation fillers are used as a polymer-based heatradiation composite material, and high heat radiation fillers having aplanar shape are advantageous. In the case of fibrous or globularfillers, the contact between fillers is not a plane contact but rather apoint contact, and thus the transfer efficiency of the lattice vibrator(phonon) may be rapidly reduced. Examples of planar high heat radiationfiller include boron nitride and graphite.

When a sample is manufactured by injection-molding a composite resindensely filled with planar particles, the planar particles are orientedin one direction by a shear force applied in the injection direction,causing anisotropy of thermal conductivity. In addition, a denselyfilled heat radiation composite material manufactured by typicalinjection molding has limitations of workability reduction due to lowresin flowability, high price of filler, and weight increase.

Hybrid fillers in which two types of filler, planar fillers and globularfillers, have a limitation in that their heat transfer efficiency cannotbe maximized due to weight increase by the thick filling of the wholefiller and relatively low thermal conductivity of globular particles.Also, the thermal energy transfer capacity is reduced due to scatteringof phonon in pores of the filled particles and the polymer resin filledin between the globular particles and planar particles.

Typical examples of heat transfer resistance factors of polymer-basedheat radiation composite materials include an interfacial resistancebetween matrix resin and filler, a resistance due to defects in filler,and a resistance occurring in a contact portion between fillers. Theseresistance factors cause the heat conduction efficiency to besignificantly reduced.

The interfacial resistance between matrix resin and filler is inassociation with the interfacial stability. For this, resin needs to befully impregnated into the surface of filler, and thus the mechanicalproperties can simultaneously increase. The resistance due to defects infiller is determined by physical factors in the selection stage and thepretreatment stage of filler. The interfacial resistance at the contactpoints between fillers may be minimized by maximizing a surface contactbetween fillers. For this, a densification process for the surfacecontact between planar particles is needed. Since the densificationprocess of planar particles is essential for the efficiency of phonontransfer, but may cause a collapse of a bulky network, the densificationprocess of planar particles needs to be induced while maintaining thenetwork of particles.

Attempts to overcome these problems have implemented a compositecontaining expanded graphite or expandable polymer. For example, apolymer/graphene nano composite material with good conductivitymanufactured by effectively dispersing graphite-based graphite materialssuch as graphene, expanded graphite, or undenatured graphite in apolymer matrix. Unfortunately, since graphite has a small amount ofpolar group on the surface thereof, it is difficult to effectivelydisperse graphite in a polymer. Also, since planar particles are notdensified, the thermal conductivity is low.

Other attempts to overcome these problems have implemented a nanocomposite with an expanded graphite/epoxy nano composite composition anda nano composite, which has excellent thermal and mechanicalcharacteristics. For example, a nano composite, that is manufactured byfusing and mixing acid-treated and heat-treated expanded graphite withepoxy resin, may have limited anisotropy due to the particle shape ofthe expanded graphite.

Accordingly, there is a need for a material that can effectively radiateheat generated in batteries to increase the lifespan and the reliabilityof a high capacity battery package for an electric vehicle, which can bedisposed between a pouch type of lithium ion batteries as an interfacialplate material or can be used for upper and lower plate covers(hereinafter, referred to as housing) for effectively coupling a fixedbattery cell and an interfacial plate module and increasing theirdurability.

SUMMARY OF THE DISCLOSURE

The present invention provides a high heat radiation composite in whichhybrid filler heat-treated after expandable polymeric beads are filledin the pore spaces formed when planar particles of expanded graphite aredispersed into matrix polymer, thereby providing a high heat radiationcomposite that can achieve the heat transfer characteristics andsimultaneously overcome thermal anisotropy.

In one exemplary embodiment, the present invention provides a high heatradiation composite containing hybrid filler, wherein the hybrid filleris formed by filling expandable polymeric beads in expanded graphite andperforming heat treatment, and is dispersed into matrix polymer.

In another exemplary embodiment, the present invention provides a methodfor manufacturing a high heat radiation composite containing hybridfiller, including: manufacturing a dispersion solution by dispersingexpandable polymeric beads in ethanol; immersing expanded graphite inthe dispersion solution in which the expandable polymeric beads aredispersed; manufacturing the hybrid filler by heat-treating thedispersion solution in which the expanded graphite is immersed to removeethanol; and manufacturing the high heat radiation composite containingthe hybrid filler through an extrusion/injection process formanufacturing a sample by dispersing the manufactured hybrid filler intomatrix polymer.

Other aspects and exemplary embodiments of the invention are discussedinfra.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present invention will now bedescribed in detail with reference to certain exemplary embodimentsthereof illustrated by the accompanying drawings, which are givenhereinbelow by way of illustration only, and thus are not limitative ofthe present invention, and wherein:

FIG. 1 is a view illustrating the shape of expandable polymeric bead inan exemplary embodiment of the present invention;

FIG. 2 is a view illustrating hybrid filler according to an exemplaryembodiment of the present invention; and

FIG. 3 is a view illustrating the microstructure of a high heatradiation composite by stages according to an exemplary embodiment ofthe present invention.

Reference numerals set forth in the Drawings includes reference to thefollowing elements as further discussed below:

-   1: expandable thermoplastic bead-   2: shape of expandable thermoplastic bead inserted into EG pores-   3: expanded graphite plate-   4: solution (ethanol)-   5: expandable thermo plastic bead before expansion-   6: expanded graphite plate-   7: EG pores in which ethanol is removed and E-beads are filled and    expanded-   8: densification of expanded graphite plate-   9: network of graphite plate expanded and densified-   10: expandable thermoplastic beads after expansion-   11: heat transfer path-   12: thermoplastic matrix

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variouspreferred features illustrative of the basic principles of theinvention. The specific design features of the present invention asdisclosed herein, including, for example, specific dimensions,orientations, locations, and shapes will be determined in part by theparticular intended application and use environment. In the figures,reference numbers refer to the same or equivalent parts of the presentinvention throughout the several figures of the drawing.

DETAILED DESCRIPTION

Hereinafter reference will now be made in detail to various embodimentsof the present invention, examples of which are illustrated in theaccompanying drawings and described below. While the invention will bedescribed in conjunction with exemplary embodiments, it will beunderstood that present description is not intended to limit theinvention to those exemplary embodiments. On the contrary, the inventionis intended to cover not only the exemplary embodiments, but alsovarious alternatives, modifications, equivalents and other embodiments,which may be included within the spirit and scope of the invention asdefined by the appended claims.

It is understood that the term “vehicle” or “vehicular” or other similarterm as used herein is inclusive of motor vehicles in general such aspassenger automobiles including sports utility vehicles (SUV), buses,trucks, various commercial vehicles, watercraft including a variety ofboats and ships, aircraft, and the like, and includes hybrid vehicles,electric vehicles, plug-in hybrid electric vehicles, hydrogen-poweredvehicles and other alternative fuel vehicles (e.g., fuels derived fromresources other than petroleum). As referred to herein, a hybrid vehicleis a vehicle that has two or more sources of power, for example bothgasoline-powered and electric-powered vehicles.

Unless specifically stated or obvious from context, as used herein, theterm “about” is understood as within a range of normal tolerance in theart, for example within 2 standard deviations of the mean. “About” canbe understood as within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%,0.1%, 0.05%, or 0.01% of the stated value. Unless otherwise clear fromthe context, all numerical values provided herein are modified by theterm “about.”

Ranges provided herein are understood to be shorthand for all of thevalues within the range. For example, a range of 1 to 50 is understoodto include any number, combination of numbers, or sub-range from thegroup consisting of 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33,34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, or 50,as well as all intervening decimal values between the aforementionedintegers such as, for example, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8,and 1.9. With respect to sub-ranges, “nested sub-ranges” that extendfrom either end point of the range are specifically contemplated. Forexample, a nested sub-range of an exemplary range of 1 to 50 maycomprise 1 to 10, 1 to 20, 1 to 30, and 1 to 40 in one direction, or 50to 40, 50 to 30, 50 to 20, and 50 to 10 in the other direction.

The above and other features of the invention are discussed infra.Hereinafter, an exemplary embodiment of the present invention will bedescribed in detail with reference to the accompanying drawings.

The present invention provides a high heat radiation compositecomprising hybrid filler, in which the hybrid filler is heat-treatedafter expandable polymeric beads are filled in expanded graphite, andthen dispersed into matrix polymer.

The expansion coefficient of the expanded graphite may range from about200 to about 400, which represents the quotient of expandable graphite'spost-heating expansion volume and original sample mass.

When the expansion coefficient is outside of this range, pores byexpansion may not be formed. More preferably, the expansion coefficientof the graphite may range from about 300 to about 350. Herein, the term“expansion” refers to a process in which a pressure is applied to theplanar particles to allow the stacked planar particles to become apartfrom each other, and the edge portions of the particles adhere to eachother to form pores three-dimensionally while intercalant filled betweenplanar particles is being evaporated by a heat-treatment process ofabout 500° C. to about 600° C.

According to an exemplary embodiment, the expanded graphite may have astacked planar structure. The expanded graphite may be manufactured byacid-treating and heat-treating the planar structure of graphite, andmay have a lower bulk density and a wider surface area than that oftypical graphite. Micro voids may be formed between planar particles,which show very excellent characteristics in hygroscopicity due to thecapillary phenomenon. The average particle size of the expanded graphitemay range from about 600 μm to about 1,500 μm, and the size of poresformed may range from about 1 μm to about 15 μm. Also, since a porousnetwork is effectively formed between planar particles, the expandedgraphite may be very advantageous for providing the heat transfercharacteristics. However, since anisotropy or pore network collapse maybe caused by a shear force during the injection process, there is alimitation in improving the thermal characteristics only with theexpanded graphite.

The expandable polymeric beads (expandable microbead; E-bead) may be inthe form of granules in which the core is a liquefied hydrocarbon andthe shell is a thermoplastic resin. The expandable polymeric bead may bemicrobeads with a mechanism in which when heated, the plastic shellportion is softened, and the core portion in a liquid state increases inpressure, allowing the particles to expand. The microbead may includevarious grades of commercial products (e.g., Expancel). The averageparticle size of the expandable polymeric beads may range from about 1μm to about 10 μm, and more preferably, from about 2 μm to about 3 μm.

The mixture ratio of the expanded graphite to the expandable polymericbeads may range from about (5 to 20):(10 to 40). When the amount of theexpanded graphite is larger than that of the expandable polymeric beads,the expandable polymeric beans may not be fully filled in the expandedgraphite, reducing the improvement of the heat transfer characteristicsdue to the expansion of beads. On the other hand, when the amount of theexpanded graphite is smaller than that of the expandable polymericbeads, the polymeric beads with low thermal conductivity may disperseinto the matrix instead of the graphite pores. Accordingly, there is alimitation in that the thermal conductivity of the compositemanufactured may be rapidly reduced.

After the expandable polymeric beads are filled into the micro spaces ofthe expanded graphite through heat treatment, the beads may be expandedby heat treatment to induce a densification process of planar particles,increasing the efficiency of phonon transfer by increasing the surfacecontact between particles and simultaneously maintaining the porenetwork structure formed in the expanded graphite.

In an exemplary embodiment, the matrix polymer may be comprised of oneselected from the group consisting of polyethylene, polypropylene,polystyrene, polyalkylene terephthalate, polyamide resin, polyacetalresin, polycarbonate, polysulfone, and polyimide.

The expanded graphite and expandable polymeric beads of the high heatradiation composite may be enlarged from the initial average particlesizes of the expanded graphite and the expandable polymeric beads to theaverage particle sizes of about 900 μm to about 2,000 μm and about 3 μmto about 5 μm, respectively. The average particle sizes may increaseabout 1.5 times compared to the initial average particles sizes.Compared to a typical PA6-based graphite composite, the thermalconductivity of the high heat radiation composite may range from about10 W/mK to about 20 W/mK in both injection and thickness directions at afilling ratio of about 40 wt % to about 50 wt %.

The high heat radiation composite according to an exemplary embodimentof the present invention may be manufactured as follows.

First, a dispersion solution may be prepared by dispersing expandablepolymeric beads in ethanol. Next, expanded graphite may be immersed inthe dispersion solution in which the expandable polymeric beads aredispersed. Next, the hybrid filler may be manufactured by heat-treatingthe dispersion solution in which the expanded graphite is immersed toremove ethanol. Next, the high heat radiation composite containing thehybrid filler may be manufactured through an extrusion/injection processfor manufacturing a sample by dispersing the hybrid filler into a matrixpolymer.

The content of the expandable polymeric beads may be about 10 wt %compared to the total weight of the ethanol and the expanded graphite.

During manufacturing of the hybrid filler, the temperature of the heattreatment may range from about 80° C. to about 120° C., and in thiscase, which expands the expandable polymeric beads to achieve adensification effect between planar particles of the expanded graphite.

In the extrusion/injection process, the content of the hybrid filler mayrange from about 40 wt % to about 50 wt % compared to the matrixpolymer. When the amount of the hybrid filler is greater than about 50wt %, the mechanical properties of the sample may be deteriorated. Onthe other hand, when the amount of the hybrid filler is smaller thanabout 40 wt %, a heat transfer path between fillers (e.g., a porestructure) may not be formed, or sufficiently formed, thereby failing toachieve a desired thermal conductivity.

In the extrusion/injection process, the injection temperature may rangefrom about 180° C. to about 280° C., so that the expandable polymericbeads may be secondarily expanded, which widens the interval betweenplates of the expanded graphite to facilitate impregnation of polymerresin into the surface of the filler. Accordingly, the interfacialstability between resin and filler may be improved, leading to animprovement of the mechanical properties of the composite and areduction of the heat transfer interfacial phenomenon between resin andfiller.

Hereinafter, the present invention will be described in more detailbased on an exemplary embodiment, but the present invention will not belimited thereto.

EXAMPLES Manufacture of Composite Containing Hybrid Filler

The following examples illustrate exemplary embodiments of the inventionand are not intended to limit the same.

Expandable polymeric beads (expandable microbead: E-bead) having aweight of about 10 g to about 40 g and a size of about 2 mm to about 3mm may be dispersed in 100 ml ethanol by stirring under mild conditions(e.g., about 100 rpm) of ordinary temperature. 5 g to 20 g expandedgraphite (EG) may be immersed in the E-bead ethanol dispersion solution,and then the dispersion solution may be incubated for about 30 minutesat room temperature (e.g., about 25° C.).

Thereafter, the dispersion solution may be heated to a temperature ofabout 80° C. or higher to remove the ethanol, and the E-bead may beexpanded to induce densification of planar particles while maintainingthe porous network. As the hybrid filler is manufactured, secondaryexpansion of E-beads dispersed in polyamide (PA6) matrix resin may occurduring the extrusion or injection process. While the pores are beingenlarged, the resin impregnation interruption phenomenon due to thecapillary phenomenon may disappear, and the impregnation of molten PA6resin into the enlarged pores of the expanded graphite may be improved.

TEST EXAMPLES Heat Conduction Characteristics of Composite ContainingHybrid Filler

With a filling ratio of about 40 wt % to about 50 wt %, the thermalconductivity of a typical PA6-based graphite composite shows about 5.0W/mK to about 8.0 W/mK in the injection direction (in-plane) and about2.0 to about 3.0 W/mK in the thickness direction (through-plane), andshows distinct anisotropic characteristics. However, with the fillingratio typically used in a conventional art composite, the expandedgraphite-E bead hybrid composite sample shows the heat conductioncharacteristics of about 10 W/mK or more in both injection and thicknessdirections.

Accordingly, as shown in FIG. 3, while the contact surface between theexpanded graphite plates is being increased by the densification processusing the E-bead, the porous network structure may be maintained, andthe heat transfer path may be effectively formed through the network,maximizing the contact area between the filler interfaces to improve theheat transfer characteristics, solving the thermal anisotropy due toglobular bead particles, and minimizing the interfacial resistancebetween filler and resin.

According to embodiments of the present invention, a high heat radiationcomposite that has excellent heat radiation characteristics compared totypical heat radiation composites and may over overcome the thermalanisotropy may be manufactured by heat treating expanded graphite andthermally expandable microbeads to form hybrid filler and dispersing thehybrid filler into matrix polymer to manufacture a composite through anextrusion/injection process.

Also, the high heat radiation composite can be utilized as a materialfor a plate disposed between pouch cells in a cell module of a batterysystem for an electric vehicle with improved reliability, safety andlifespan, and a material for a battery case and housing.

The invention has been described in detail with reference to exemplaryembodiments thereof. However, it will be appreciated by those skilled inthe art that changes may be made in these embodiments without departingfrom the principles and spirit of the invention, the scope of which isdefined in the appended claims and their equivalents.

What is claimed is:
 1. A high heat radiation composite, comprising: ahybrid filler; and a matrix polymer, wherein the hybrid filler is formedby placing expandable polymeric beads into microvoids or micro spaces ofporous expanded graphite and heat-treating the resulting mixture, andthe hybrid filler is dispersed into the matrix polymer, thereby formingthe composite.
 2. The composite of claim 1, wherein the expandedgraphite is graphite expanded by acid treatment and heat treatment. 3.The composite of claim 1, wherein the expanded graphite has an expansioncoefficient that ranges from about 200 to about
 400. 4. The composite ofclaim 1, wherein the expanded graphite has an average particle size thatranges from about 600 μm to about 1,500 μm.
 5. The composite of claim 1,wherein the expandable polymeric beads have an average particle sizethat ranges from about 1 μm to about 10 μm.
 6. The composite of claim 1,wherein the mixture ratio of the expanded graphite to the expandablepolymeric beads ranges from about 5-20: about 10-40.
 7. The composite ofclaim 1, wherein the matrix polymer is selected from the groupconsisting of polyethylene, polypropylene, polystyrene, polyalkyleneterephthalate, polyamide resin, polyacetal resin, polycarbonate,polysulfone, and polyimide.
 8. The composite of claim 1, wherein theexpanded graphite has an average particle size that ranges from about900 μm to about 2,000 μm.
 9. The composite of claim 1, wherein theexpanded polymeric beads have an average particle size that ranges fromabout 3 μm to about 5 μm.
 10. The composite of claim 1, wherein thecomposite has a thermal conductivity of about 10 W/mk to about 20 W/mk.